Østberg competed in his first rally in 2004 and made his WRC debut at the 2006 Swedish Rally, where he finished 31st in a Subaru Impreza WRC 2003. That season he also started the Rally Finland and the Wales Rally GB, retiring from the former and finishing 23rd in the latter.
In 2007, Østberg continued with an Impreza WRC 2005 through the Prodrive-supported Adapta team, entering six world rallies. He recorded his first WRC stage win on a super special stage at the Swedish Rally and secured his first championship point with an eighth-place finish at the Rally Finland. He also won the Norwegian Rally Championship title that year.
The 2008 and 2009 seasons saw Østberg develop steadily within the Adapta programme. In 2009 he scored points on three WRC rounds and achieved his best result to that point — a sixth place at the Rally Portugal. By 2010, still competing primarily in an Impreza-based car alongside occasional outings in a Ford Fiesta S2000, he placed 11th in the drivers' standings for a second consecutive year.
Østberg moved to Stobart M-Sport Ford for 2011, driving a Ford Fiesta RS WRC. The season marked a significant step forward: he ran at the front of the 2011 Rally Sweden and finished second, then repeated the result at the 2011 Rally GB, the final round of the season. He ended the year sixth in the championship.
The 2012 season brought Østberg's defining moment. Returning to Adapta Motorsport but retaining the Fiesta RS WRC, he placed third in Sweden before winning at the Rally de Portugal — his first career WRC victory — after the leading Mikko Hirvonen was disqualified from the results. The win made Østberg the first non-French or non-Finnish driver to win a WRC event since 2005 and only the second Norwegian in history to stand on the top step of a world rally podium. He followed the victory with a third place in Argentina and ended the season fourth in the overall standings.
In 2013, Østberg joined Qatar M-Sport — effectively the leading Ford-aligned squad following the American manufacturer's official withdrawal from factory involvement — secured two podium finishes, and finished sixth in the championship.
Citroën signed Østberg for 2014, pairing him with Kris Meeke in the Citroën World Rally Team. He collected four podiums that year and finished fifth in the championship. In 2015, his second season with Citroën, he opened strongly with two runner-up finishes in the opening four rounds and briefly ran second in the championship. He ended the year fourth in the standings — characterised in commentary as the best result outside the dominant Volkswagen trio of drivers.
For 2016, Østberg returned to M-Sport, this time with Ola Fløene as his co-driver in an evolution-spec Fiesta RS WRC. He took two third-place finishes and ended the season seventh.
Østberg announced a reduced schedule for 2017, sharing a private entry with Martin Prokop, citing the birth of his first child. His career has been defined by consistent front-running pace in machinery that rarely matched the fastest factory entries outright, and he remains one of the most respected privateer-to-works narratives in the modern WRC era.